The present invention relates to absorbers for photothermal conversion and more particularly to semiconductor absorbers for photothermal conversion.
Photothermal converters comprise absorbers which absorb sunlight and convert it into heat by raising the temperature of the absorber. Potential applications include conversion of sunlight into heat for generation of steam and electricity. Heretofore, absorbers were made from a material that was as little light-reflecting and as little transparent to light as possible, i.e. as dark as possible and preferably black. However, a dark body also radiates energy as a function of its temperature. In fact, the darker a body is, the more energy it radiates with increasing temperature of the body. Thus, for an absorber, there is an equilibrium temperature at which the total amount of energy absorbed equals the energy which is radiated. The absorber would then operate at this equilibrium temperature, since above this temperature the body would radiate more energy than is absorbed. For most dark bodies, the equilibrium temperature is fairly low. However, to be useful and economical for generation of steam and electricity, the absorber must operate at a relatively high temperature, for example, at about 750.degree.K.
U.S. Pat. No. 3,000,375 discloses a homogeneous semiconductor absorber which is intended to raise the equilibrium temperature. The absorber comprises a homogeneous layer of semiconductor, such as tellurium, silicon or germanium, on a reflector. Tellurium suffers from the drawback of cost, while homogeneous silicon suffers from the drawback of reflecting too much of the incident solar radiation. Homogeneous germanium suffers from having a low bandgap thereby preventing operation of temperatures suitable for economical generation of steam for electricity.